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<title>Offsetof (Using the GNU Compiler Collection (GCC))</title>

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<h3 class="section" id="Support-for-offsetof"><span>6.53 Support for <code class="code">offsetof</code><a class="copiable-link" href="#Support-for-offsetof"> &para;</a></span></h3>
<a class="index-entry-id" id="index-_005f_005fbuiltin_005foffsetof"></a>

<p>GCC implements for both C and C++ a syntactic extension to implement
the <code class="code">offsetof</code> macro.
</p>
<div class="example smallexample">
<pre class="example-preformatted">primary:
        &quot;__builtin_offsetof&quot; &quot;(&quot; <code class="code">typename</code> &quot;,&quot; offsetof_member_designator &quot;)&quot;

offsetof_member_designator:
          <code class="code">identifier</code>
        | offsetof_member_designator &quot;.&quot; <code class="code">identifier</code>
        | offsetof_member_designator &quot;[&quot; <code class="code">expr</code> &quot;]&quot;
</pre></div>

<p>This extension is sufficient such that
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define offsetof(<var class="var">type</var>, <var class="var">member</var>)  __builtin_offsetof (<var class="var">type</var>, <var class="var">member</var>)
</pre></div>

<p>is a suitable definition of the <code class="code">offsetof</code> macro.  In C++, <var class="var">type</var>
may be dependent.  In either case, <var class="var">member</var> may consist of a single
identifier, or a sequence of member accesses and array references.
</p>
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